This invention relates to the fabrication of optical fibres and in particular to silica-based optical fibre fabrication employing a rod-in-tube method of manufacture.
In the type of rod-in-tube method of manufacture of optical fibre with which the invention is particularly, but not exclusively concerned, a rod preform, comprising a rod of glass forming the core material of a fibre and provided with a core-cladding structure is placed inside a sleeve tube of glass, which serves to increase the cross-sectional area of the composite. The rod preform and tube assembly is then lowered into a furnace. In the hot zone the tube collapses onto the rod preform forming a single entity which is then drawn down into a fibre. There are many factors which influence the quality of the resultant fibre including assembly advance rate, fibre draw rate, furnace temperature, furnace hot zone profile, and start rod and tube geometry. The rod-in-tube method serves to extend the fibre yield of a rod preform by adding to it the mass of the sleeve tube. To achieve the correct core to O/D (outside diameter) ratio in the final fibre, the optical material in the rod preform is conventionally grown to be oversize, so that a sleeve tube of a given cross-sectional area will reduce it to the correct geometry in the final fibre.